This invention relates in general to electrophotographic imaging members and more specifically, to imaging members having an improved acid doped charge transport layer and process for fabricating the imaging members.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate or photoreceptor is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a charge generating layer, and a charge transport layer. Either the charge generating layer or the charge transport layer may be located adjacent the conductive layer. The charge transport layer can contain an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. Although excellent toner images may be obtained with such multilayered photoreceptors, it has been found that acid doping of the charge transport layer enhanced predictability of performance for high precision copiers, duplicators and printers having narrow sensitivity windows. This acid doping is described, for example in U.S. Pat. No. 4,725,518, U.S. Pat. No. 5,149,612 and U.S. Pat. No. 5,356,741, the entire disclosures of these patents being incorporated herein by reference. This acid doping overcame the unpredictable variations in electrical performance of photoreceptors made from commercially available methylene chloride and polycarbonate that contained impurities that fluctuated from batch to batch and from the batch to batch variabilities of the generator layer pigment. Acid doping is preferably accomplished by combining transport layer solutions from two different pots (one doped with a very low amount of acid and the second doped with a higher concentration of acid) are mixed just prior to the introduction of the coating solution into the coating die. The amount of material from the second pot is adjusted continuously to bring electrical characteristics to the desired level. Surprisingly, with the passage of time, the optimum amount of acid used for doping diminished to about 3 ppm based on the weight of methylene chloride, due, probably, to unknown material and/or process changes pertaining to synthesis of the commercially available methylene chloride solvent and/or other components in the charge transport layer such as the polycarbonate film forming binder.
As doping was reduced to lower levels, the resulting photoreceptors began to exhibit "edge spikes" in which some regions of the photoreceptors have higher background potential (lower sensitivities) resulting in dark background print out in these regions. The loss in sensitivity along the edges occurs in a periodic pattern. The edge spike becomes less prominent if the doping acid, such as trifluoroacetic acid (TFA), concentration is increased to more than about 10 ppm, based on the weight of methylene chloride. However, when the concentration of TFA is increased to more than about 20 ppm in the photoreceptors, the photoreceptors show increased depletion, higher dark decay and long term cyclic instability.
Thus it is desirable to have a quality control tool such as acid doping that can be varied during the manufacturing and yet have the acid concentration stay between about 5 and about 15 ppm based on the weight of methylene chloride.